The retina is a very small tissue lining the back inside surface of the eye. It is only 0.25 to 0.3 mm thick but 15 sq cm in area. In western countries, disease of the retina is the main cause of untreatable blindness. There is a vital need to be able to deliver biological treatments or operations to precisely determined retina locations and at specific depths, such as into blood vessels or to particular cells of the retina with a precision better than 5 microns. Currently there is no way such biological treatments or operations can be achieved with such accuracy, thus hindering specific drug and other treatments of the retina. The alternative of delivering drugs through the systemic circulation is not possible when only a small region of the retina is targeted, and delivery of powerful drugs into the ocular contents rather than at a particular location in the retina can have unwanted effects.
Current ocular ultramicrosurgical operations, where it is sought to perform delicate manipulations on areas of tissue as small as a few microns in diameter, have had a limited success rate due to the inability of surgeons to accurately control surgical tools using manual manipulations under the microscope. Even the steadiest hand has an unavoidable physiological tremor which at rest has an amplitude of about 50 micrometers and a frequency of between 7 and 12 cycles per second. After 30 minutes of activity, this physiological tremor increases to an amplitude of 2 to 5 mm at a frequency between 4 and 6 cycles per second.
In ocular research laboratories, retinal arterial or venous occlusion has been treated (mainly in animals) by in vivo cannulation of the vessel and injection of clot-clearing agents such as tissue plasminogen activator (tPA), [ Allf and de Juan Jr 1987]), but application of the technique in routine surgery on humans has been prevented by the very low success rate of such operations, typically 20 percent or lower. The low success rate is due in the case of arterial or vein occlusions to the damage done by the surgeon to the blood vessel when micro cannulation is attempted, the micro cannulation device being relatively substantial (typically 20 to 50 microns) compared with the size of the blood vessel (typically about 100 microns).
Hunter et al [ Hunter et al 1994] have described a sophisticated teleoperated microsurgical robot adapted to automation of corneal and lens operations. This system is not adapted to automation of ultramicrosurgical retinal operation.
Manual systems which assist the eye surgeon, particularly in animal experimentation, have been known for a number of years and incorporate stereotactic systems to support surgical tools such as micropipettes in a manner such that the tool shaft is orientated about a pivot point coincident with the point of entry of the tool into the ocular cavity at the pars plana. [ Toth et al 1992, Benner et al 1993]. Such systems do not completely isolate the physiological tremor from the tip of the surgical tool, and since they are manual in nature result in time consuming operations, reducing the practicality of routine application to human surgery.